NPT thread are tapered and are different than standard bolt because the diameter of the threads change as they go deeper into the hole. Using a drill bit that is too large for the diameter of the hole will result in the threads being too shallow for the application. These shallow threads will not create a proper mechanical seal, and the threads will leak fluid.
If the drill bit or the tap is a correct size for the workpiece, the threads will be of the appropriate depth to create an mechanical seal once the threads are tapped. The taper of the NPT threads will create an interference fit that becomes even tighter with the application of more torque to the tapped threads. There are different types of threads.
How to Tap NPT Threads
The National Pipe Taper (NPT) is one of them. The other types of threads include the NPTF and the BSP threads. The NPTF threads, often called Dryseal threads, is designed to create a seal by crushing the threads together.
Unlike NPT threads, which require a sealant to prevent fluid leaks, NPTF threads dont require a sealant. The BSP threads are a different standard of threads that are extensively used in Europe. BSP threads are different than NPT threads.
For any given workpiece material, there is a specific type of tap that must be used. Depending on the threading application, there are three different types of taps to choose from: taper taps, plug taps, and bottoming taps. Taper taps are used to start a tapped hole.
The gradual slope of a taper tap allow for better alignment of the tap to the workpiece. Plug taps cut more of the depth of the thread than taper taps do. Bottoming taps are used to tap threads in blind holes.
To tap a hole properly, it is best to start with a taper tap and then switch to a plug tap or a bottoming tap if the thread depth is greater then the taper tap will provide. Using the wrong type of taps will cause them to snap. Depending on the material of the workpiece, there is a specific type of lubricant that must be used with the tap.
The lubricant prevents the tap from damaging the workpiece and the tap. If the workpiece material is stainless steel, the risk of the metal working hardens (creates a hardened layer) if the tap rubs against the metal without cutting it. Therefore, to tap stainless steel, you should use the chlorinated fluids in the lubricant.
For carbon steel, the sulfurized oils should be used in the lubricant. For aluminum, the light mineral oil or kerosene should be used in the lubricant. This is because aluminums flute will readily stick to the metal chips if they are not lubricated properly.
For cast iron, no lubricant is required because the metal contains graphite that act as a lubricant. To tap a hole properly, the tap should be turned in a specific movement to avoid clogging of the tap with the metal chips. The tap should be turned half a turn forward.
Then, it should be turned back off one quarter of a turn. The reason for this movement is to break the metal chips that are produced during the tapping process. If the metal chips are not broken with this movement, the chips will clog the tap and could potentially break the tap.
To ensure that the threads are properly tapped, a gauge should be used. The tool that is used to measure the thread is called a plug gauge. The plug gauge ensure that the taper of the thread is properly sized before the assembly of the system that contains the tapped thread.
If the tapped threads do not hand thread in for a few turns when attempting to join two components with the tapped threads, the threads is incorrect. Using a plug gauge ensures that the threads are correctly tapped so that the system will not leak under pressure.
